[dm-devel] [PATCH 1/7] multipath: fix tur checker locking

[Bart Van Assche]

On Wed, 2018-02-07 at 16:49 -0600, Benjamin Marzinski wrote:
> Commit 6e2423fd fixed a bug where the tur checker could cancel a
> detached thread after it had exitted. However in fixing this, the new
> code grabbed a mutex (to call condlog) while holding a spin_lock. To
> deal with that, and to try to keep with the maixim "lock data, not
> code", I've changed how the tur checker synchronizes with its thread.
> 
> Now, the tur checker creates joinable threads, and detaches them when
> the thread is finished or has timed out. To track the state of the
> threads, I've added a new variable to the checker context, ct->attached.
> When a thread starts, attached is set to 1. When the thread finishes, it
> saves the value of attached, and then zeros it out, while locked. If
> attached was set, it detaches itself.
> 
> When the tur checker gives up on a thread, it also saves and decrements
> ct->attached, while locked. At the same time it saves the value of
> ct->thread.  If attached was set, it cancels the thread, and then
> detaches it.
> 
> So the values that are protected by the spin lock are now ct->holders,
> ct->thread, and ct->attached. There are cases where the tur checker just
> wants to know if the thread is running. If so, its safe to simply read
> ct->thread without locking.  Also, if it knows that the thread isn't
> running, it can freely access all of these variables. I've left the
> locking in-place in these cases to make the static analyzers happy.
> However I have added comments stating when the locking isn't actually
> necessary.

Hello Ben,

Have you considered to move the condlog() statements out of the spinlock
section? I think that would lead to a much smaller and less contrived change
than the patch you posted.

Thanks,

Bart.